diff --git a/target/s390x/misc_helper.c b/target/s390x/misc_helper.c
index 83d38944d7..eca82441d0 100644
--- a/target/s390x/misc_helper.c
+++ b/target/s390x/misc_helper.c
@@ -288,7 +288,9 @@ void HELPER(diag)(CPUS390XState *env, uint32_t r1, uint32_t r3, uint32_t num)
     switch (num) {
     case 0x500:
         /* KVM hypercall */
+        qemu_mutex_lock_iothread();
         r = s390_virtio_hypercall(env);
+        qemu_mutex_unlock_iothread();
         break;
     case 0x44:
         /* yield */